In internal combustion engines, tank venting systems are known wherein the fuel vapors developing on account of and in dependence on specific parameters (fuel temperature, fuel quantity, vapor pressure, air pressure, scavenging quantity, et cetera) are not merely vented off into atmosphere and are instead directed into the engine. Conventionally, this is accomplished by providing an intermediate storage filled with, for example, activated carbon which receives the fuel vapors developing, for example, when the vehicle is stationary and directs them to the intake area of the engine via a conduit. In this connection, it is further known to prevent or minimize increased exhaust emissions which may occur as a result of such an additional air-fuel mixture attributable to tank venting by releasing the tank venting function only under specific operating conditions of the engine. For the foregoing, reference can be made to the publication of Robert Bosch GmbH entitled "Motronic"--Technische Beschreibung C5/1 of August 1981 and German published patent application DE-OS No. 2,829,958.
The intermediate storage container accommodating the activated carbon filter can store fuel vapors up to a specific maximum amount, with the filter being scavenged during engine operation by the vacuum pressure generated by the engine in the intake ducting for which the filter has an opening to the atmosphere. As a result, even if scavenging of the intermediate storage unit is only permitted under specific operating conditions, tank venting necessarily produces an additional air-fuel mixture which, being either not measured or not measurable at reasonable expense, tampers with the fuel metering signal, that is, in a fuel injection system, the duration of the injection control instruction t.sub.i, which is normally computed in a complex procedure with a very high degree of accuracy, and tampers with the resultant fuel quantity supplied to the internal combustion engine. Such an additional fuel quantity which affects particularly also the driveability under specific conditions and which, in extreme cases, may consist of almost 100% vented air or 100% vented fuel vapor, is not acceptable, not even if the impact of this disturbance is directly related to the intake pressure developed by the internal combustion engine by means of pneumatic final controlling elements, nor if an electronic on/off control is provided which cuts off the supply of the tank venting mixture completely in the presence of particularly sensitive operating conditions, such as at idling.